Thomas Zurbuchen, NASA's associate administrator for the ScienceMission Directorate, said that these discoveries should be viewed as Mars "telling us to stay the course and keep searching for evidence of life". That's because the surface of Mars is constantly bombarded with radiation that can break down organic compounds. On Earth, such carbon-rich compounds are one of life's cornerstones. They could even have been transported from elsewhere in the solar system.

Curiosity rover on Mars. Methane is another organic molecule.

Inorganic carbon is carbon that is found in compounds that are completely unlike biological molecules. On Mars, that's been a maddening challenge: While scientists have detected bursts of methane on the planet, they've appeared at random - and thus, it's been hard to figure out what the source is.

Christopher Webster, an atmospheric science research fellow at NASA's Jet Propulsion Laboratory in California, said it is possible existing microbes are contributing to the Martian atmospheric methane. But the origin of methane on Mars has always been debated. Before, researchers couldn't understand why the little bit of methane detected in the Martian atmosphere varied.

The full findings will also appear in the 8 June edition of the journal Science. During the summer months, levels of the gas detected by Curiosity rose to about 0.7 parts per billion; in winter, they fell to roughly half that. This is the period when the southern icecap - which freezes carbon dioxide out of the atmosphere, but not methane - is at its biggest, so enhanced methane is not unexpected. "It's an astounding observation".

Previously, it was an open question whether signs of life might be preserved on the now-harsh Martian surface.

Webster theorizes the methane created either now or long ago is seeping from deep underground reservoirs up through cracks and fissures in the crust.

But what's making the methane? "Or is it something that is stored from an ancient time that's being slowly released?" In a companion article, an outside expert describes the findings as "breakthroughs in astrobiology". "It's not cold and dead-it's perhaps hovering right on the edge of habitability". Over time, a picture of the ebb and flow of methane on Mars has emerged.

"I'm equally as fascinated by the idea that life never got started on Mars in the first place".

Future missions will help.

In the meantime Curiosity has undertaken what Webster calls "the most important measurements of Mars methane made to date". But the planned ExoMars 2020 rover, part of a joint mission of the European Space Agency and the Russian space agency Roscosmos, will have a drill that can reach a depth of about 6.5 feet.

In addition to finding organic molecules in the rocks in Gale Crater, rover scientists are reporting another intriguing finding.